1. Field of the Invention
The present invention relates to a substrate voltage generating circuit and a semiconductor integrated circuit device, and particularly relates to a substrate voltage generating circuit, which uses transistor having thin gate oxide films, as well as a semiconductor integrated circuit device.
2. Description of the Background Art
Present dynamic random access memories, which will be referred to as "DRAMs" hereinafter, use constant power supply voltages. However, the DRAM is internally provided with a substrate voltage generating circuit, which is arranged on a chip and generates a negative voltage for the purposes of (1) preventing a pn junction in the chip from being forwardly biased in a minimum manner, (2) reducing a change in threshold voltage of a MOS transistor due to a substrate effect, (3) increasing a threshold voltage of a parasitic MOS, (4) reversely biasing and thereby reducing a junction capacity, and others.
A structure of a substrate voltage generating circuit 700 in the prior art will be described below with reference to FIG. 16. Referring to FIG. 16, a substrate voltage generating circuit 700 includes a ring oscillator 702 and a charge pump 704.
Ring oscillator 702 includes inverters 71, 72 and 73. Charge pump circuit 704 includes a capacity element C70 and a PMOS transistor Q70. Capacity element C70 receives a clock signal issued from oscillator 702. PMOS transistor Q70 is connected between capacity element C70 and a substrate voltage output node OUT. A charge pump operation is repeated based on the output of oscillator 702 so that electrons (VBB) are supplied to a substrate (not shown).
In recent years, the power supply voltages have been increasingly lowered. This is because lowering of the operation voltages is unavoidably required due to lowering of transistor breakdown voltages, which is caused by miniaturization of transistors. Accordingly, it is demanded to provide a charge pump circuit of a boost type, which uses a low power supply voltage and has high pump efficiency.
In particular, a large substrate current occurs during accessing of a device so that such a circuit is required that supplies a large current commensurate with it and provides a predetermined negative voltage (substrate voltage) VBB.
Meanwhile, a gate oxide film thickness tox of transistors has been reduced in accordance with scaling of devices. If a charge pump circuit of a boost type is used, an intensity of an electric field applied to a channel of a transistor increases. This results in extreme increase in energy of carriers moving through the channel, and therefore extremely increases a possibility of generation of hot carriers. Hot carriers thus generated cause shifting of a threshold voltage and lowering of a mutual conductance, and therefore causes a problem that device characteristics are deteriorated over time. This impairs the reliability.